The present invention was made as a further development of the valve shown and described in the copending U.S. Pat. application of McGee, Ser. No. 800,437, filed May 25, 1977, now U.S. Pat. No. 4,138,091 for a drive nut plate assembly for the valve, and the copending U.S. Pat. application of McGee, et al, Ser. No. 800,438, filed May 25, 1977, now U.S. Pat. No. 4,149,558 for a selective back seat for the valve. A parallel development is disclosed in the copending U.S. Pat. application of McGee, Ser. No. 955,232 filed Oct. 27, 1978. This latter application describes a valve operator that is provided with a back seat which comes into use when the normally non-rising stem operator and its bearing-enclosing bonnet gland are raised, e.g. so that stem packing can be replaced, and is further provided with another way to actuate the back seat, one which will come into play should the valve be subjected to heat of sufficient intensity as to call the integrity of the stem packing or other critical parts into question. The further activation mode depends on the melting of fusible material which permits the operator and its bearings to rise relative to the bonnet.
All the foregoing, in turn, represent developments of valves described in the following prior U.S. patents.
______________________________________ Patentee U.S. Pat. No. Issue Date ______________________________________ Crain, et al 3,349,789 Oct. 31, 1967 McGee 3,789,875 Feb. 5, 1974 Boitnott 3,990,679 Nov. 9, 1976 ______________________________________
Others have worked with fusible materials in order to provide safety devices for valves. Examples are shown in the following prior U.S. patents:
______________________________________ Patentee U.S. Pat. No. Issue Date ______________________________________ Wicke 3,771,540 Nov. 13, 1973 Allen 3,788,600 Jan. 29, 1974 Wicke 3,842,854 Oct. 22, 1974 Wicke 3,896,835 July 29, 1975 ______________________________________